The present invention relates to a cable, in particular a cable for electric power transmission or distribution or for telecommunications. More particularly, the present invention relates to a cable as defined above comprising at least one coating layer and having self-repairing protection which is capable of restoring the continuity of the coating layer after it has been broken.
Electrical cables, in particular low- or medium-voltage cables for the distribution of electric energy for domestic or industrial use, generally consist of one or more conductors individually insulated by a polymeric material and coated with a protective sheath, which is also made of a polymeric material. These cables, particularly when installed underground, in tunnels or inside buried pipes, are subject to damage on these layers caused by various types of mechanical abuses, for example accidental impact with sharp tools such as shovels or picks, which exert on the cable both cutting and compression action. This can lead to partial or total rupture of the outer sheath and possibly also of the insulating layer, with consequent infiltration of moisture and generation of leakage currents. If the rupture of the coating layers reaches the conductor, the combined effect of leakage currents and moisture leads to a gradual corrosion of the conductor until, at the utmost, to a complete breakage of the conductor itself.
To obtain effective protection against such mechanical abuses, the cable can be provided with an outer structure capable of withstanding both cutting and compression, this outer structure consisting, for example, of a sheath made of a metal or of a plastic material combined with metal armouring. Besides being expensive, this solution leads to a considerable increase in cable dimensions and rigidity, thus making this solution unsuitable for cables which require easiness of installation and low costs, such as, in particular, in the case of low-voltage cables.
In patent application DE-1,590,958 a telecommunications or high-current cable is described which is protected from mechanical damage by means of an outer sheath having, on its inside, microcapsules containing a liquid which is capable of solidifying rapidly once the microcapsule has been broken. To this purpose, it is mentioned as preferred the use of the two components commonly used for manufacturing expanded polyurethane, these components being microencapsulated separately so that they react together at the moment the microcapsules are broken, forming an expanded material which closes the accidental cut. Alternatively, it is possible to use liquids which solidify when placed in contact with external agents, for example with moisture.
According to the Applicant, the solution proposed in the above-mentioned patent application is difficult to implement in practice and has many drawbacks. Firstly, it is to be observed that the possibility of self-repairing is limited to the outer sheath, and no indications regarding the possibility of restoring integrity of the insulating layer are provided. Moreover, to obtain an effective self-repairing effect, it is necessary to introduce a large amount of microencapsulated material during sheath extrusion, and this operation can result to be extremely difficult, besides being expensive. Lastly, it is to be pointed out that the mechanism of action of the microcapsules is irreversible, consequently the self-repairing effect can be carried out only once, namely at the moment the microcapsules are broken. Actually, during the various stages of the cable life (manufacturing, storage, installation, use), the coating layers are inevitably subjected to external mechanical actions of compression and bending and to thermal cycles of expansion and compression, which can lead to rupture of the microcapsules with consequent expansion and/or solidification of the material contained therein. Therefore, this material will no longer be able to effect the desired self-repairing action when the sheath should actually be damaged. It is also to be noted that, even when microcapsules are used containing a liquid material which solidifies on contact with moisture, accidental rupture of the microcapsules without any actual damage to the outer sheath nonetheless leads to solidification of the material because residual moisture is always present inside the cable.
The Applicant has now found that, in consequence of a mechanical damage which creates a discontinuity in at least one of the cable coating layers, it is possible to obtain effective self-repairing of the coating by virtue of the presence of an inner layer, placed, for example, between the insulating layer and the outer sheath. This inner layer comprises a material having a predetermined cohesiveness and, at the same time, a controlled flowability, which is capable of repairing the damage by restoring the continuity of the coating layer. After creation of a discontinuity in the coating, the material xe2x80x9cmovesxe2x80x9d towards the point of damage and fills up, at least partly, the discontinuity by forming a substantially continuous layer which is capable of maintaining the functionality of the cable under the expected working conditions. The action of the self-repairing material, which occurs with a reversible mechanism, prevents, among other things, moisture infiltration and establishment of leakage currents, and thus a quick corrosion of the conductor.
The flowability of the material is predetermined so as to have sufficient fluidity at the working temperature of the cable, and at the same time so as to prevent the material from draining from the cable extremities or leaking in an uncontrolled manner from the coating rupture point.
In a first aspect, the present invention thus relates to a cable comprising a conductor and at least one coating layer, characterized in that the said cable comprises an inner layer comprising a self-repairing material having a predetermined cohesiveness and a controlled flowability.
According to a preferred aspect, the cable according to the present invention comprises an insulating coating layer and an outer sheath, and is characterized in that the inner layer is placed between the insulating layer and the outer sheath.
According to another embodiment of the present invention, the inner layer is placed between the conductor and the insulating layer.
According to a further embodiment, the cable according to the present invention comprises at least two insulating coating layers and is characterized in that the inner layer is placed between two of the said insulating layers.
According to a further aspect, the present invention relates to a method for imparting to a cable comprising a conductor and at least one coating layer a capacity of self-repairing the coating layer, characterized in that the said method comprises providing the cable with an inner layer comprising a material having the capacity, upon creation of a discontinuity in the coating layer, of re-establishing the continuity in the coating layer in a reversible manner.